Carbon nanotubes are considered to be one of the strongest materials in existence, with individual single-wall tubes demonstrating breaking strengths of about 120 GPa and a Young's modulus of 1 TPa. CNTs used commercially in yarns and sheets have an average theoretical tensile strength for a single CNT of about 25 GPa. This type of nano-scale performance has yet to be duplicated on macro-scale carbon nanotube assemblies, such as composite parts made up predominantly of CNT sheets and yarns.
CNT sheets and yarns are currently being made with promising macro-scale strength properties—breaking strengths of sheets exceeding 1 GPa and yarns approaching 2 GPa. Composites made from CNT sheets have breaking strengths of around 2 GPa, which exceeds the performance of most graphite composites on a bulk basis. In addition, the strain-to-failure is around 10%. (The resin component does not add significant strength properties to the CNTs in the CNT/resin composites.) Thus, CNT material: (1) approaches the strength of Kevlar®—with significantly greater fracture toughness; (2) equals the environmental robustness of graphite; and (3) is far lighter weight than either Kevlar or graphite.
CNTs function best in tension. Real structures suffer from shear and compression failures well ahead of CNT theoretical limits. Analysis reveals signs of van der Waals force failure, indicating a weak link in holding individual nanotubes together. Experiments confirm that CNT yarn failure appears to be excessive pullout/slippage from nanotube ends or flaws, not breakage of an individual nanotube.
The unique mechanical properties of CNTs have led to recent research and development of high performance CNT-polymer composites. Numerous approaches have been taken for utilizing the significant mechanical properties of CNTs to produce enhanced strength composites.
Techniques used in the past include methods for fabricating CNT yarn for improving the strength of the yarns used as the reinforcing material in polymer matricies, or for improving the ways in which CNTs are incorporated into the various polymer materials.
To date, one problem has been scaling up the strength and stiffness of the individual CNT yarns. A growing area of research involves spinning CNTs into flexible higher strength yarns. Since bundles of CNTs tend to slide past each other, owing to the weak van der Waals forces, one prior art process involves CNT yarns strengthened by exposing bundles of CNT fibers to high-energy electron radiation to create covalent bonds between individual nanotubes. [Nature.com/news/21 Mar. 2011; “A neat trick for strengthening carbon nanotube yarns”] In another process, an assembly of one or more spun yarns comprising CNTs are chemically interlinked one to another and arranged in spiral configurations in the form of a yarn, thread or fabric used as a reinforcing material in a composite structure. [US 2009/0282802 to Cooper et al.]
Various techniques have been developed for incorporating CNTs into resinous matrix composites to improve mechanical properties. In one process, highly aligned CNTs formed as a yarn are combined with a polymer resin after being stressed through a dry spinning process. The resin is cured and polymerized with the CNT structure acting as a reinforcement in a CNT-polymer composite. [Tran, et al., “Manufacturing Polymer/Carbon Nanotubes Composite Using a Novel Direct Process, Nanotechnology, vol. 22, no. 14, 2011]
In another process the CNTs are infused into the base material: a polymer, other carbon fibers, a polymer foam, or other structural material. US 2010/0276072 to Shah, et al. is one example. One prior art approach has been to enhance the physical bonding of the CNTs to the base material of the composite. [US 2001/0159270 to Davis, et al.] Other approaches chemically bond the polymer material, in which the CNTs are embedded, to the outer walls of the CNT material. [US 2010/0119822 to Hwang, et al.]
Some prior art techniques involve infiltration of the CNTs using chemical functionalization of the CNTs, to produce useful CNT-polymer composites. [U.S. Pat. No. 7,601,421 to Khabashesku, et al.] Another process for chemically modifying CNTs is described in US 2010/0256290 to Costanzo, et al.
Thus, the prior art continues to seek improvements in taking advantage of the unique properties of CNTs in order to produce commercial-scale enhanced strength composites.
The present invention provides a process for improving the bonding between individual CNT yarns and sheets that leads to greatly improved macro-scale performance properties of the CNT materials and their composites. One aspect of the invention is based on the recognition that some resins have such high viscosity that infusion pressures are insufficient to penetrate smaller spaces between the CNTs. Some resins have insufficient functional groups that do not provide the “wetting” that enhances penetration into the smallest void spaces. Some resins have molecular structures that are physically too large to penetrate CNT ropes or yarns. And some resins do not provide functional attack throughout the structure, which can undertreat the inner regions of the CNT yarns.
The present invention overcomes these drawbacks by providing a resin infusion process in which CNT yarns and sheets are infused and bonded with a low viscosity nano-resin that has yielded enhanced strength CNT yarns and sheets which are useful as structural components in a variety of composite structures. In accordance with one embodiment of the process provided by the invention, the nano-resin is essentially completely infused into the void spaces between adjacent CNT yarns, to fill the spaces while undergoing a curing process that results in greatly enhanced strength CNT yarns and sheets and the composite materials in which they are used.